This invention relates to a swing type power unit for a two-wheeled motor vehicle having a four-cycle engine.
A swing type power unit is known for a two-wheeled motor vehicle having a two-cycle engine. The vehicle does not have a rear fork, but its rear wheel is supported directly on the power unit. No such power unit is, however, available as yet for a two-wheeled motor vehicle having a four-cycle engine, because of the difficulty involved in the location of the oil pan, since a lubricant oil pump is usually employed for the engine.
According to the swing type power unit, the engine has a center approximately coinciding with that of the vehicle body to improve the operability, stability and riding comfort of the vehicle. This arrangement, however, creates between the output of a crankshaft and a crank bearing a dead space corresponding to the gap in which the rear wheel is mounted.
Oil is usually supplied to a clutch or transmission by an oil bath for "splash lubrication", or an oil pump or the like for "forced lubrication". According to the "splash lubrication method", oil is agitated for delivery to the area to be lubricated, and the agitation of the oil gives rise to a power loss. If the depth by which the clutch or transmission is immersed in the oil is reduced to decrease such power loss, the effect of lubrication is correspondingly reduced. Therefore, it is difficult to establish an appropriate oil level which can also vary with a change in the posture of a person on the vehicle.